This invention relates to a melt processable thermoplastic polymer composition that employs a non-aliphatic (e.g., a non-hydrocarbon, aromatic, or a non-hydrocarbon/aromatic polymer) non-fluorinated host polymer and a fluoropolymer.
For any melt processable thermoplastic polymer composition, there exists a critical shear rate above which the surface of the extrudate becomes rough and below which the extrudate will be smooth. See, for example, R. F. Westover, Melt Extrusion, Encyclopedia of Polymer Science and Technology, Vol. 8, pp 573-81 (John Wiley and Sons 1968). The desire for a smooth extrudate surface competes, and must be optimized with respect to, the economic advantages of extruding a polymer composition at the fastest possible speed (i.e. at high shear rates).
Some of the various types of extrudate roughness and distortion (also sometimes referred to as melt defects) observed in high and low density polyethylenes are described by A. Rudin, et al., Fluorocarbon Elastomer Aids Polyolefin Extrusion, Plastics Engineering, March 1986, at 63-66. The authors state that for a given set of processing conditions and die geometry, a critical shear stress exists above which polyolefins such as linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and polypropylene suffer melt defects. At low shear rates, defects may take the form of xe2x80x9csharkskinxe2x80x9d, a loss of surface gloss, that in more serious manifestations appears as ridges running more or less transverse to the extrusion direction. At higher rates, the extrudate can undergo xe2x80x9ccontinuous melt fracturexe2x80x9d becoming grossly distorted. At rates lower than those at which continuous melt fracture is first observed, LLDPE and HDPE can also suffer from xe2x80x9ccyclic melt fracturexe2x80x9d, in which the extrudate surface varies from smooth to rough. The authors state further that lowering the shear stress by adjusting the processing. conditions or changing the die configuration can avoid these defects to a limited extent, but not without creating an entirely new set of problems. For example, extrusion at a higher temperature can result in weaker bubble walls in tubular film extrusion, and a wider die gap can affect film orientation.
There are other problems often encountered during the extrusion of thermoplastic polymers. They include a build up of the polymer at the orifice of the die (known as die build up or die drool), excessive backpressure during extrusion runs, and excessive degradation or low melt strength of the polymer due to high extrusion temperatures. These problems slow the extrusion process either because the process must be stopped to clean the equipment or because the process must be run at a lower speed.
Certain fluorocarbon processing aids are known to partially improve the melt processability of extrudable thermoplastic hydrocarbon polymers and allow for faster, more efficient extrusion. U.S. Pat. No. 3,125,547 to Blatz, for example, first described the use of fluorocarbon polymer process aids with melt-extrudable hydrocarbon polymers wherein the fluorinated polymers are homopolymers and copolymers of fluorinated olefins having an atomic fluorine to carbon ratio of at least 1:2 and wherein the fluorocarbon polymers have melt flow characteristics similar to that of the hydrocarbon polymers.
U.S. Pat. No. 5,397,829 to Morgan et al. describes the use of copolymers of tetrafluoroethylene and hexafluoropropylene having high hexafluoropropylene content as processing aids in polyolefins.
U.S. Pat. No. 5,464,904 to Chapman et al. discloses the use of semicrystalline fluoroplastics such as copolymers of tetrafluoroethylene and hexafluoropropylene and terpolymers of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride with a polyolefin. The only enhancement of melt-processability described in this patent is shown in Examples 19 and 25 where a concentration of 1000 ppm of the fluoropolymer in linear low density polyethylene is said to reduce the extrusion pressure of the extrudable composition. There is no showing of a reduction in melt defects.
U.S. Pat. No. 5,710,217 to Blong at al. discloses an extrudable thermoplastic hydrocarbon composition that comprises an admixture of a melt processable hydrocarbon polymer as the major component and an effective amount of a chemically-resistant fluoropolymer process aid. The fluoropolymer contains at least 50% by weight of fluorine and comprises one or more fluoromonomers that are essentially completely ethylenically unsaturated.
U.S. Pat. No. 5,587,429 to Priester discloses a three part processing aid system for polyolefins. The system comprises a fluoropolymer, a polar-side-group-containing adjuvant, and a poly(oxyalkylene) polymer.
U.S. Pat. Nos. 4,904,735 and 5,013,792 (Chapman, Jr. et al.) describe a fluorinated processing aid for use with a difficultly melt-processable polymer comprising (1) a fluorocarbon copolymer which at the melt-processing temperature of the difficultly melt-processable polymer is either in a melted form if crystalline, or is above its glass transition temperature if amorphous, and (2) at least one tetrafluoroethylene homopolymer or copolymer of tetrafluoroethylene and at least one monomer copolymerizable therewith wherein the mole ratio is at least 1:1, and which is solid at the melt-processable temperature of the difficultly melt-processable polymer.
U.S. Pat. Nos. 5,064,594 to Priester et al., and U.S. Pat. No. 5,132,368 to Chapman, Jr. et al. describe the use of certain fluoropolymer process aids containing functional polymer chain end groups including xe2x80x94COF, xe2x80x94SO2F, xe2x80x94SO2Cl, SO3M, xe2x80x94OSO3M, and xe2x80x94COOM, wherein M is hydrogen, a metal cation, or a quaternary ammonium cation for use with a difficultly melt-processable polymer. These patents each require that the fluoropolymer comprise a molten component and a solid component at the extrusion temperature.
U.S. Pat. Nos. 5,015,693 and 4,855,013 to Duchesne and Johnson disclose the use of a combination of a poly(oxyalkylene) polymer and a fluorocarbon polymer as a processing additive for thermoplastic hydrocarbon polymers. The poly(oxyalkylene) polymer and the fluorocarbon polymer are used at such relative concentrations and proportions as to reduce the occurrence of melt defects during extrusion. Generally the concentration of the fluoropolymer is present at a level of from 0.005 to 0.2 weight percent of the final extrudate and the poly(oxyalkylene) polymer is present at a level of from 0.01 to 0.8 weight percent of the final extrudate. Preferably, the weight of the fluorocarbon polymer in the extrudate and the weight of the poly(oxyalkylene) polymer in the extrudate are in a ratio of 1:1 to 1:10.
EP 0 503 714 Al discloses a polyamide composition comprising
A) 100 parts by weight of a polyamide and
B) 0.001 to 5 parts by weight of a processing agent selected from fluorinated polymers. These compositions are said to exhibit improved processability as determined by a reduction in the amount of increase in extrusion pressure over time. Polyvinylidene fluoride is the only fluoropolymer exemplified in the publication.
While these known additives may provide improved melt processability in olefin polymers, they have not proven to be particularly successful in non-aliphatic polymers. Accordingly, there still exists a need for an effective processing aid to be used with non-aliphatic polymers.
It has been discovered that a certain class of fluoropolymers are surprisingly effective in improving the melt processability of non-aliphatic polymers. The improvement in melt processability can be seen in one or more ways. For example, it may be seen in a reduction of melt defects such as sharkskin in non-aliphatic polymers, or in the postponement of the occurrence of these defects to higher extrusion rates than can be typically achieved without the use of the fluoropolymer. Alternatively, it has been discovered that the fluoropolymers are also surprisingly effective in reducing the occurrence of die build up and/or reducing the amount of backpressure during extrusion of non-aliphatic polymers, and permitting the use of lower extrusion temperatures to achieve an equivalent throughput.
In one aspect, the present invention provides a novel melt processable polymer composition that comprises a major amount (i.e., at least (and preferably greater than) 50% by weight) of a melt processable thermoplastic non-aliphatic host polymer and a minor, but effective, amount of the fluoropolymer processing aid. The fluoropolymer comprises up to (and preferably less than) 50% by weight of the melt processable polymer composition. The fluoropolymer may be selected from the group consisting essentially of amorphous and partially crystalline fluoropolymers.
In a particularly preferred aspect, the present invention provides an extrudable composition that comprises a fluoropolymer processing aid that is resistant to reaction with basic, acidic, or amine-containing moieties in the host polymer or the extrudable composition. These novel compositions utilize a fluoropolymer processing aid that does not readily react with or degrade in, the presence of these moieties. These fluoropolymer processing aids contain 15% by weight or less of interpolymerized units derived from a monomer that produces an acidic hydrogen on the backbone of the resulting fluoropolymer after polymerization. Preferably, these fluoropolymer processing aids contain 10% by weight or less (more preferably 5% by weight or less) of interpolymerized units derived from a monomer that produces an acidic hydrogen on the backbone of the resulting fluoropolymer after polymerization. Most preferably the fluoropolymer processing aids are essentially free of interpolymerized units derived from a monomer that produces an acidic hydrogen on the backbone of the resulting fluoropolymer after polymerization.
In yet another aspect, the present invention provides a method for improving the melt processability of the host polymer. In this method the host polymer is mixed with an effective amount of the fluoropolymer. The resulting melt processable polymer composition is preferably mixed until there is a uniform distribution of the fluoropolymer in the host polymer. The polymer composition is then melt processed.
As used herein, an effective amount of the fluoropolymer is that which (a) reduces the occurrence of melt defects during extrusion of the host polymer below the level of melt defects occurring during the extrusion of a host polymer that does not employ the fluoropolymer, or (b) delays the onset of the occurrence of such defects to a higher extrusion rate (that is a higher shear rate), or (c) reduces the occurrence of die build up, therefore extending the time between cleanup steps, or (d) reduces backpressure, therefore providing faster throughput or allowing the use of lower extrusion temperatures.